I'm glad you, blasphemously, mentioned Zelazny. I wanted to love LORD OF LIGHT so much but I found it dense and pretentious. But not the worst sf book ever -
Zelazny's ROADMARKS is pretty awful.

Furthur blasphemy; Arthur C Clarke's 3001 THE FINAL ODYSSEY. I was grimacing the whole way through, it was so contrived and just stuck together.
2001 is excellent, 2010 very good and 2061 just nice and readable - What is it about series? Do they invariably slip downhill?

Here`s an obscure candidate ; RETURN TO MARS by Captain W.E. Johns who wrote all the Biggles books. A cliche-infected and insipid piece of pseudo-sf written in the 50's when I suppose sf was fashionable for a while or something. Dreadful stuff.

Rumfuddle, augh... you mentioned Lord of Light. Yeah, I tried to read that. It included the sentence... read no further, ye of weak stomach...

"That's when the fit hit the Shan."

I put the book down shortly after, since I didn't really get the plot or the characters anyway. I have a co-worker who considers this light, escapist fiction, so I know some people grok the Z-man... just not me. Missing the appropriate neuron or something.

Yeah, what awful dialogues.
I was reminded of those sects who try to come up with their own holy scriptures and frame them in authentic sounding scriptural language. I actually finished it, coz' I'm like that - have to finish things. I see it as a chance to practice speed reading or something *g* And I loved the premise. An it did have it,s moments, a certain atmosphere....but really, I think it's just badly written; graceless, difficult and artificial.
Zelazny's supposed to be brainy, literary SF. I do not think we're stupid tho'.
Can someone jump on this thread and defend Zelazny please. That'd be interesting.

This is a good tread because you can get a better idea whether to take advice from people in the discussion if you know what they like and what they don't like.
Don't take my advice if you don't like Zelazny. He's my favorite author by far.
I'm not going to defend him. If you don't get it, you don't get it.

Not necesarily a good idea.
I love Samuel R Delaney and Ursula le Guin, for examle, who were among those who began publishing around the same time as Zelazny in the 60's and are sometimes refered to as having been part of a similar trend in SF towards more 'literary' values, compared to the 'hard' science of previous generations.
I like so many 'New Wave' era authors in general I'd generally pay attention to a Zelazny fan's recommendations. Much more attention than I'd pay to someone who only reads and raves about Star Trek novels for example.
One of the most exciting, and indeed inexplicable, things SF is the way opinions differ so wildly.

Farnham's Freehold by RAH
He is one of my favourites but I couldn't even finish this one and I tried three times. It put me off RAH for almost a year.
It's funny, I started to read sf because of RAH (Puppet Masters, Sixth Column, and Have Spacesuit Will Travel).

If I remember right I liked Have Spacesuit will Travel but it was a book written before Heinlein started writing serious adult fiction. As far as worst SF goes Battlefield Earth was pretty bad. I didn't much care for any of the Dune books either. But either one was at least distinguished enough that I remember it. I have probably read a hundred SF books that I picked up and read the first 20 pages and put them back down again thinking "Ugghh, what a waste of a perfectly good half and hour, I could have been cutting my toenails or something" Those I don't even remember titles.

Worst ever, hey? Well there was this little novel called Privateer (can't remember author) which I found pretty bad. Some friends of mine liked it but I didn't because it just seemed a little too space opera-ish (space opera is fine but I found that in the novel it was absolutely heaped on).

If you want to find the very worst, try some Star Trek novels. Some are okay, most are mediocre, a few are incredibly bad. Also try some of Larry Niven's late books, like the last two Ringworld ones. Oh, and the Rama sequels. The original Rendezvous with Rama was quite good, but the sequels, I'm pretty sure, are really entirely by Gentry Lee and have Clarke's name on them as an advertisement. I actually read all the way to the end because I wanted the Answer--but they never give it to you! They just throw out all of the original mysteries in favor of something entirely different and much worse. There are actually some interesting things in the sequels, but they never get developed enough.

As for Heinlein, I liked his books, including Have Spacesuit Will Travel, but I have not read any of them since sixth grade or so. I'm kind of afraid to go back, for fear of what I will find.